Positive ignition engines cause combustion of a hydrocarbon and air mixture using spark ignition. Contrastingly, compression ignition engines cause combustion of a hydrocarbon by injecting the hydrocarbon into compressed air. Positive ignition engines can be fuelled by gasoline fuel, gasoline fuel blended with oxygenates including methanol and/or ethanol, liquid petroleum gas or compressed natural gas.
A three-way catalyst (TWC) typically contains one or more platinum group metals, particularly those selected from the group consisting of platinum, palladium and rhodium.
TWCs are intended to catalyse three simultaneous reactions: (i) oxidation of carbon monoxide to carbon dioxide, (ii) oxidation of unburned hydrocarbons to carbon dioxide and water; and (iii) reduction of nitrogen oxides to nitrogen and oxygen. They are not designed to adsorb NOx from lean exhaust gas. Reactions (i)-(iii) inclusive occur most efficiently when the TWC receives exhaust gas from an engine running at or about the stoichiometric point. As is well known in the art, the quantity of carbon monoxide (CO), unburned hydrocarbons (HC) and nitrogen oxides (NOx) emitted when gasoline fuel is combusted in a positive ignition (e.g. spark-ignited) internal combustion engine is influenced predominantly by the air-to-fuel ratio in the combustion cylinder. An exhaust gas having a stoichiometrically balanced composition is one in which the concentrations of oxidising gases (NOx and O2) and reducing gases (HC and CO) are substantially matched. The air-to-fuel ratio that produces this stoichiometrically balanced exhaust gas composition is typically given as 14.7:1.
Theoretically, it should be possible to achieve complete conversion of O2, NOx, CO and HC in a stoichiometrically balanced exhaust gas composition to CO2, H2O and N2 (and residual O2) and this is the duty of the TWC. Ideally, therefore, the engine should be operated in such a way that the air-to-fuel ratio of the combustion mixture produces the stoichiometrically balanced exhaust gas composition.
A way of defining the compositional balance between oxidising gases and reducing gases of the exhaust gas is the lambda (λ) value of the exhaust gas, which can be defined according to equation (1) as:Actual engine air-to-fuel ratio/Stoichiometric engine air-to-fuel ratio,  (1)wherein a lambda value of 1 represents a stoichiometrically balanced (or stoichiometric) exhaust gas composition, wherein a lambda value of >1 represents an excess of O2 and NOx and the composition is described as “lean” and wherein a lambda value of <1 represents an excess of HC and CO and the composition is described as “rich”. It is also common in the art to refer to the air-to-fuel ratio at which the engine operates as “stoichiometric”, “lean” or “rich”, depending on the exhaust gas composition which the air-to-fuel ratio generates: hence stoichiometrically-operated gasoline engine or lean-burn gasoline engine.
It should be appreciated that the reduction of NOx to N2 using a TWC is less efficient when the exhaust gas composition is lean of stoichiometric. Equally, the TWC is less able to oxidise CO and HC when the exhaust gas composition is rich. The challenge, therefore, is to maintain the composition of the exhaust gas flowing into the TWC at as close to the stoichiometric composition as possible.
Of course, when the engine is in steady state it is relatively easy to ensure that the air-to-fuel ratio is stoichiometric. However, when the engine is used to propel a vehicle, the quantity of fuel required changes transiently depending upon the load demand placed on the engine by the driver. This makes controlling the air-to-fuel ratio so that a stoichiometric exhaust gas is generated for three-way conversion particularly difficult. In practice, the air-to-fuel ratio is controlled by an engine control unit, which receives information about the exhaust gas composition from an exhaust gas oxygen (EGO) (or lambda) sensor: a so-called closed loop feedback system. A feature of such a system is that the air-to-fuel ratio oscillates (or perturbates) between slightly rich of the stoichiometric (or control set) point and slightly lean, because there is a time lag associated with adjusting air-to-fuel ratio. This perturbation is characterised by the amplitude of the air-to-fuel ratio and the response frequency (Hz).
The active components in a typical TWC comprise one or both of platinum and palladium in combination with rhodium, or even palladium only (no rhodium), supported on a high surface area oxide, and an oxygen storage component.
When the exhaust gas composition is slightly rich of the set point, there is a need for a small amount of oxygen to consume the unreacted CO and HC, i.e. to make the reaction more stoichiometric. Conversely, when the exhaust gas goes slightly lean, the excess oxygen needs to be consumed. This was achieved by the development of the oxygen storage component that liberates or absorbs oxygen during the perturbations. The most commonly used oxygen storage component (OSC) in modern TWCs is cerium oxide (CeO2) or a mixed oxide containing cerium, e.g. a Ce/Zr mixed oxide.
Ambient PM is divided by most authors into the following categories based on their aerodynamic diameter (the aerodynamic diameter is defined as the diameter of a 1 g/cm3 density sphere of the same settling velocity in air as the measured particle):
(i) PM-10—particles of an aerodynamic diameter of less than 10 μm;
(ii) Fine particles of diameters below 2.5 μm (PM-2.5);
(iii) Ultrafine particles of diameters below 0.1 μm (or 100 nm); and
(iv) Nanoparticles, characterised by diameters of less than 50 nm.
Since the mid-1990's, particle size distributions of particulates exhausted from internal combustion engines have received increasing attention due to possible adverse health effects of fine and ultrafine particles. Concentrations of PM-10 particulates in ambient air are regulated by law in the USA. A new, additional ambient air quality standard for PM-2.5 was introduced in the USA in 1997 as a result of health studies that indicated a strong correlation between human mortality and the concentration of fine particles below 2.5 μm.
Interest has now shifted towards nanoparticles generated by Diesel and gasoline engines because they are understood to penetrate more deeply into human lungs than particulates of greater size and consequently they are believed to be more harmful than larger particles, extrapolated from the findings of studies into particulates in the 2.5-10.0 μm range.
Size distributions of Diesel particulates have a well-established bimodal character that correspond to the particle nucleation and agglomeration mechanisms, with the corresponding particle types referred to as the nuclei mode and the accumulation mode respectively (see FIG. 1). As can be seen from FIG. 1, in the nuclei mode, Diesel PM is composed of numerous small particles holding very little mass. Nearly all Diesel particulates have sizes of significantly less than 1 μm, i.e. they comprise a mixture of fine, i.e. falling under the 1997 US law, ultrafine and nanoparticles.
Nuclei mode particles are believed to be composed mostly of volatile condensates (hydrocarbons, sulfuric acid, nitric acid etc.) and contain little solid material, such as ash and carbon. Accumulation mode particles are understood to comprise solids (carbon, metallic ash etc.) intermixed with condensates and adsorbed material (heavy hydrocarbons, sulfur species, nitrogen oxide derivatives etc.). Coarse mode particles are not believed to be generated in the Diesel combustion process and may be formed through mechanisms such as deposition and subsequent re-entrainment of particulate material from the walls of an engine cylinder, exhaust system, or the particulate sampling system. The relationship between these modes is shown in FIG. 1.
The composition of nucleating particles may change with engine operating conditions, environmental condition (particularly temperature and humidity), dilution and sampling system conditions. Laboratory work and theory have shown that most of the nuclei mode formation and growth occur in the low dilution ratio range. In this range, gas to particle conversion of volatile particle precursors, like heavy hydrocarbons and sulfuric acid, leads to simultaneous nucleation and growth of the nuclei mode and adsorption onto existing particles in the accumulation mode. Laboratory tests (see e.g. SAE 980525 and SAE 2001-01-0201) have shown that nuclei mode formation increases strongly with decreasing air dilution temperature but there is conflicting evidence on whether humidity has an influence.
Generally, low temperature, low dilution ratios, high humidity and long residence times favour nanoparticles formation and growth. Studies have shown that nanoparticles consist mainly of volatile material like heavy hydrocarbons and sulfuric acid with evidence of solid fraction only at very high loads.
Contrastingly, engine-out size distributions of gasoline particulates in steady state operation show a unimodal distribution with a peak of about 60-80 nm (see e.g. FIG. 4 in SAE 1999-01-3530). By comparison with Diesel size distribution, gasoline PM is predominantly ultrafine with negligible accumulation and coarse mode.
Particulate collection of Diesel particulates in a Diesel particulate filter is based on the principle of separating gas-borne particulates from the gas phase using a porous barrier. Diesel filters can be defined as deep-bed filters and/or surface-type filters. In deep-bed filters, the mean pore size of filter media is bigger than the mean diameter of collected particles. The particles are deposited on the media through a combination of depth filtration mechanisms, including diffusional deposition (Brownian motion), inertial deposition (impaction) and flow-line interception (Brownian motion or inertia).
In surface-type filters, the pore diameter of the filter media is less than the diameter of the PM, so PM is separated by sieving. Separation is done by a build-up of collected Diesel PM itself, which build-up is commonly referred to as “filtration cake” and the process as “cake filtration”.
It is understood that Diesel particulate filters, such as ceramic wallflow monoliths, may work through a combination of depth and surface filtration: a filtration cake develops at higher soot loads when the depth filtration capacity is saturated and a particulate layer starts covering the filtration surface. Depth filtration is characterized by somewhat lower filtration efficiency and lower pressure drop than the cake filtration.
WO 03/011437 discloses a gasoline engine having an exhaust system comprising means for trapping PM from the exhaust gas and a catalyst for catalysing the oxidation of the PM by carbon dioxide and/or water in the exhaust gas, which catalyst comprising a supported alkali metal. The means for trapping PM is suitable for trapping PM of particle range 10-100 nm, and can be a wallflow filter made from a ceramic material of appropriate pore size such as cordierite coated with the catalyst, a metal oxide foam supporting the catalyst, a wire mesh, a Diesel wallflow filter designed for Diesel applications, an electrophoretic trap or a thermophoretic trap (see e.g. GB-A-2350804).
WO 2008/136232 A1 discloses a Diesel particulate filter comprising a honeycomb filter having a cell wall composed of a porous cell wall base material and, provided on its inflow side only or on its inflow and outflow sides, a surface layer and satisfying the following requirements (1) to (5): (1) the peak pore diameter of the surface layer is identical with or smaller than the average pore diameter of the cell wall base material, and the porosity of the surface layer is larger than that of the cell wall base material; (2) with respect to the surface layer, the peak pore diameter is from 0.3 to less than 20 μm, and the porosity is from 60 to less than 95% (measured by mercury penetration method); (3) the thickness (L1) of the surface layer is from 0.5 to less than 30% of the thickness (L2) of the cell wall; (4) the mass of the surface layer per filtration area is from 0.01 to less than 6 mg/cm2; and (5) with respect to the cell wall base material, the average pore diameter is from 10 to less than 60 μm, and the porosity is from 40 to less than 65%. See also SAE paper no. 2009-01-0292.
Other techniques suggested in the art for separating gasoline PM from the gas phase include vortex recovery.
Emission legislation in Europe from 1 Sep. 2014 (Euro 6) requires control of the number of particles emitted from both Diesel and gasoline (positive ignition) passenger cars. For gasoline EU light duty vehicles the allowable limits are: 1000 mg/km carbon monoxide; 60 mg/km nitrogen oxides (NOx); 100 mg/km total hydrocarbons (of which ≦68 mg/km are non-methane hydrocarbons); and 4.5 mg/km particulate matter ((PM) for direct injection engines only). Although the authorities have not set the PM number standard for Euro 6 yet, it is widely understood that it will be set at 6.0×1011 per km. The present specification is based on the assumption that this number will be adopted in due course.
In the United States, no similar emission standards have been set. However, the State of California Air Resources Board (CARB) recently published a paper entitled “Preliminary Discussion Paper—Amendments to California's Low-Emission Vehicle [LEV] Regulations for Criteria Pollutants—LEV III” (release date 8 Feb. 2010) in which a new PM standard of between 2 and 4 mg PM/mile (1.25-2.50 mg PM/km (currently 10 mg PM/mile (6.25 mg PM/km))) is proposed, the paper commenting that: “Staff has received input from a number of manufacturers suggesting that a standard of 3 mg PM/mile (1.88 mg PM/km) can be met for gasoline direct injection engines without requiring the use of particulate filters.” Additionally, the paper states that since the PM mass and count emissions appear to be correlated: “Although a mandatory number standard is not being considered at this time, an optional PM number standard of about 1012 particles/mile [6.2511 particles/km] is being considered (which could be chosen by manufacturers instead of the PM mass standard)”. However, since neither the PM standard nor the PM number standard has been set by CARB yet, it is too soon to know whether particulate filtration will be necessary for the Californian vehicle market or the US vehicle market generally. It is nevertheless possible that certain vehicle manufacturers will choose filters in order to provide a margin of safety on any positive ignition engine design options selected to meet whatever standards are eventually set.
The new Euro 6 emission standard presents a number of challenging design problems for meeting gasoline emission standards. In particular, how to design a filter, or an exhaust system including a filter, for reducing the number of PM gasoline (positive ignition) emissions, yet at the same time meeting the emission standards for non-PM pollutants such as one or more of oxides of nitrogen (NOx), carbon monoxide (CO) and unburned hydrocarbons (HC), all at an acceptable back pressure, e.g. as measured by maximum on-cycle backpressure on the EU drive cycle.
It is envisaged that a minimum of particle reduction for a three-way catalysed particulate filter to meet the Euro 6 PM number standard relative to an equivalent flowthrough catalyst is ≧50%. Additionally, while some backpressure increase for a three-way catalysed wallflow filter relative to an equivalent flowthrough catalyst is inevitable, in our experience peak backpressure over the MVEG-B drive cycle (average over three tests from “fresh”) for a majority of passenger vehicles should be limited to <200 mbar, such as <180 mbar, <150 mbar and preferably <120 mbar e.g. <100 mbar.
As previously mentioned, PM generated by positive ignition engines has a significantly higher proportion of ultrafine, with negligible accumulation and coarse mode compared with that produced by Diesel (compression ignition) engines, and this presents challenges to removing it from positive ignition engine exhaust gas in order to prevent its emission to atmosphere. In particular, since a majority of PM derived from a positive ignition engine is relatively small compared with the size distribution for Diesel PM, it is not practically possible to use a filter substrate that promotes positive ignition PM surface-type cake filtration because the relatively low mean pore size of the filter substrate that would be required would produce impractically high backpressure in the system.
Furthermore, generally it is not possible to use a conventional wallflow filter, designed for trapping Diesel PM, for promoting surface-type filtration of PM from a positive ignition engine in order to meet relevant emission standards because there is generally less PM in positive ignition exhaust gas, so formation of a soot cake is less likely; and positive ignition exhaust gas temperatures are generally higher, which can lead to faster removal of PM by oxidation, thus preventing increased PM removal by cake filtration. Depth filtration of positive ignition PM in a conventional Diesel wallflow filter is also difficult because the PM is significantly smaller than the pore size of the filter medium. Hence, in normal operation, an uncoated conventional Diesel wallflow filter will have a lower filtration efficiency when used with a positive ignition engine than a compression ignition engine.
Another difficulty is combining filtration efficiency with a washcoat loading, e.g. of catalyst for meeting emission standards for non-PM pollutants, at acceptable backpressures. Diesel wallflow particulate filters in commercially available vehicles today have a mean pore size of about 13 μm. However, we have found that washcoating a filter of this type at a sufficient catalyst loading such as is described in US 2006/0133969 to achieve required gasoline (positive ignition) emission standards can cause unacceptable backpressure.
In order to reduce filter backpressure it is possible to reduce the length of the substrate. However, there is a finite level below which the backpressure increases as the filter length is reduced. Suitable filter lengths for filters according to the present invention are from 2-12 inches long, preferably 3-6 inches long. Cross sections can be circular and in our development work we have used 4.66 and 5.66 inch diameter filters. However, cross-section can also be dictated by space on a vehicle into which the filter is required to fit. So for filters located in the so-called close coupled position, e.g. within 50 cm of the engine exhaust manifold where space is at a premium, elliptical or oval filter cross sections can be contemplated. As would be expected, backpressure also increases with washcoat loading and soot loading.
There have been a number of recent efforts to combine TWCs with filters for meeting the Euro 6 emission standards.
US 2009/0193796 discloses an emission treatment system downstream of a gasoline direct injection engine for treatment of an exhaust gas comprising hydrocarbons, carbon monoxide, nitrogen oxides and particulates, the emission treatment system comprising a catalysed particulate trap comprising a three-way conversion (TWC) catalyst coated onto or within a particulate trap. The general description says that a TWC-coated particulate trap can be located downstream of a first TWC catalyst, where the first TWC catalyst can be smaller than otherwise required due to the TWC functionality coated onto the particulate trap. No specific example is provided of an exhaust system comprising both a first TWC and a downstream TWC-coated particulate trap.
WO 2009/043390 discloses a catalytically active particulate filter comprising a filter element and a catalytically active coating composed of two layers. The first layer is in contact with the in-flowing exhaust gas while the second layer is in contact with the out-flowing exhaust gas. Both layers contain aluminium oxide. The first layer contains palladium, the second layer contains an oxygen-storing mixed cerium/zirconium oxide in addition to rhodium. In Examples, a wallflow filter substrate of unspecified mean pore size is coated with a first layer at a loading of approximately 31 g/l and a second layer at a loading of approximately 30 g/l. That is, the washcoat loading is about 1.00 g in−3. For a majority of vehicle applications, this coated filter is unlikely to be able to meet the required emission standards alone. The specification also discloses an emission control system for the purification of exhaust gases of combustion engines operated with a predominantly stoichiometric air/fuel mixture, containing a catalytically active particle filter according to the disclosure. There is no teaching or suggestion in WO '390 that the emission control system can be used in combination with a separate TWC disposed on a flow-through substrate monolith and located upstream of the catalytically active particle filter.
Our GB 2468210 discloses a filter for filtering particulate matter (PM) from exhaust gas emitted from a positive ignition engine, which filter comprising a porous substrate having inlet surfaces and outlet surfaces, wherein the inlet surfaces are separated from the outlet surfaces by a porous structure containing pores of a first mean pore size, wherein the porous substrate is coated with a washcoat comprising a plurality of solid particles wherein the porous structure of the washcoated porous substrate contains pores of a second mean pore size, and wherein the second mean pore size is less than the first mean pore size. In embodiments, the washcoat is catalysed and in a particular embodiment the catalyst is a TWC.
A practical difficulty with filters for filtering particulate matter from positive ignition internal combustion engines is that the space on a passenger vehicle can be limited, and the filter may have to be located in a cooler, so-called “underfloor” location, slung beneath the vehicle underbody. Often, there is insufficient space on a vehicle to locate a filter in a hotter location any closer to the engine manifold (the so-called “close-coupled” location) and in any event the relatively high thermal mass of the filter may prevent the catalyst from “lighting off” sufficiently quickly following engine cold-start. This is important because legislation assesses vehicle emissions over a defined drive cycle. Most of a positive ignition engine vehicle's pollutant emissions occur on the test cycle in the first tens of seconds following cold-start. If a TWC is not active quickly enough following cold-start, this can mean the difference between passing and failing a relevant emissions standard over the drive cycle as a whole.
“Light-off” may be defined as the temperature at which a catalyst catalyses a reaction at a desired conversion activity. For example “CO T50” is a temperature at which a particular catalyst causes the conversion of carbon monoxide in a feed gas, for example to CO2, with at least 50% efficiency. Similarly, “HC T80” is the temperature at which hydrocarbon, perhaps a particular hydrocarbon such as octane or propene, is converted, e.g. to water vapour and to CO2 at 80% efficiency or greater.
In practice, therefore, exhaust systems for vehicular positive ignition engines currently on the market include a relatively low thermal mass TWC comprising a flow-through monolith substrate located in the close coupled position.